Powershell treats empty string as equivalent to null in switch statements but not if statements
Powershell auto-magically casts a $null to an empty string. Consequently, when using $null on a .NET API call, Powershell actually casts it to an empty string. To pass an actual null value in an API call, use [NullString]::Value instead.
I think is a bug of powershell 2.0 (here some info on MSFT Connect).
I can say that in v 3.0 you code return out here!
The following statements show that $null is not equivalent to the empty string in a switch statement.
$a=""; switch ($a){$null {"null"} "" {"empty string"}}$a=$null; switch ($a){$null {"null"} "" {"empty string"}}